1. Field of the Invention
This invention relates to a method of and a system for reading out an image, a solid image sensor, and an image detecting sheet.
2. Description of the Related Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, they store a part of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image read-out method or a radiation image read-out system in which a sheet provided with a layer of the stimulable phosphor (will be referred to as “a stimulable phosphor sheet”, hereinbelow) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet so that the stimulable phosphor sheet emits stimulated emission as signal light bearing thereon information on the radiation image, and the stimulated emission is photoelectrically detected, thereby obtaining an image signal bearing thereon a radiation image of the object. Further there have been known various image read-out systems which are different in the manner of scanning the stimulable phosphor sheet with the stimulating light beam, the form of the means for photoelectrically detecting the stimulated emission, or the like.
For example, there has been known an image read-out method and an image read-out system which include a stimulating light source which emits spot light like a laser beam as the stimulating light, a photomultiplier as a zero-dimensional photoelectric convertor which converts stimulated emission emitted from the stimulable phosphor sheet upon exposure to the spot light to an electric signal and a stimulating light scanning optical system which causes the spot light to scan the stimulable phosphor sheet in a main scanning direction while moving the spot light and the photomultiplier in a sub-scanning direction relatively to the stimulable phosphor sheet, and in which stimulated emission emitted from parts of the stimulable phosphor sheet upon exposure to the spot light is read in sequence by the photomultiplier. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12492 and 56(1981)-11395.
The photomultiplier comprises a photocathode face and an electron multiplier portion, and is excellent in that since a weak signal generated by weak stimulated emission is amplified by an external photo electric effect and accordingly, the electric signal obtained by the photomultiplier is less affected by electric noise. It is preferred that the photocathode face of the photomultiplier is high in sensitivity to the stimulated emission in a wavelength range of about 300 to 500 nm (in a blue region) and low in sensitivity to the stimulating light in a wavelength range of about 600 to 700 nm (in a red region).
Further the photomultiplier may be have a circular or polygonal photocathode face or an elongated photocathode face extending in a length substantially equal to the width of the stimulable phosphor sheet, and is used as a zero-dimensional sensor in either case. In the former case, the photomultiplier is employed together with a light guide which is provided with an elongated light inlet end face extending in a length substantially equal to the width of the stimulable phosphor sheet and a light exit end face connected to the circular or polygonal photocathode face of the photomultiplier.
However, when such a photomultiplier is used, the following problems arise.    (1) The photomultiplier comprises a vacuum glass tube, and accordingly is fragile.    (2) The photomultiplier comprises a complicated multistage diode for electron multiplication, and accordingly is difficult to reduce the thickness. Further, a long photomultiplier such as one 17 inches in length is expensive.    (3) The photocathode employing an external photo electric effect is low in quantum efficiency to the stimulated emission in a wavelength range of about 300 to 500 nm (in a blue region) and normally about 10 to 20%, whereas the quantum efficiency of the photocathode to the stimulating light in a wavelength range of about 600 to 700 nm (in a red region) is relatively high and normally about 0.1 to 2%. Accordingly, a special stimulating light cut filter is required to a sufficient S/N ratio, which adds to the cost.    (4) The photomultiplier comprises a complicated multistage dynode, and accordingly it is difficult to make a line sensor which is large in width, e.g., 17 inches, and is as small as about 100 μm in picture element size.
From the viewpoint of shortening the stimulated emission reading time, reduction of the size of the system and reduction of the manufacturing cost of the system, there have been proposed, for instance, in Japanese Unexamined Patent Publication No. 60(1985)-111568, an image read-out method and system in which a line stimulating light source such as a fluorescent lamp, a cold cathode fluorescent lamp, or a LED array which projects a line beam onto the stimulable phosphor sheet, a line sensor having a solid photoelectric convertor element array extending in the direction of length of the portion of the stimulable phosphor sheet exposed to the line beam and a scanning means which moves the line source and the line sensor relatively to the stimulable phosphor sheet in a sub-scanning direction substantially perpendicular to the portion of the stimulable phosphor sheet exposed to the line beam are employed, and stimulated emission emitted from parts of the stimulable phosphor sheet exposed to the line beam is read in sequence while moving the line source and the line sensor relatively to the stimulable phosphor sheet in the sub-scanning direction.
In the above identified Japanese patent publication, there is disclosed, as the solid photoelectric convertor element for forming the line sensor, a photoconductor including those whose band gaps E are either larger or smaller than energy of photons hc/λ at the wavelength λ of the stimulating light (E>hc/λ, or E<hc/λ). Those whose band gaps E are larger than energy of photons hc/λ at the wavelength λ of the stimulating light include, for instance, ZnS, ZnSe, CdS, TiO2 and ZnO, and those whose band gaps E are smaller than energy of photons hc/λ at the wavelength λ of the stimulating light include, for instance, a-SiH, CdS(Cu), ZnS(Al), CdSe and PbO, a-representing “amorphous”. Further, it has been proposed to use a line sensor formed of Si photodiodes.
However use of a line sensor formed of the materials described above gives rise to the following problems. That is, though it is advantageous that the solid photoelectric convertor element itself has electron multiplying function since the stimulated emission is very weak, any one of the line sensors formed of the materials described above except the Si photodiode exhibits no avalanche amplification effect as the electron multiplying function. On the other hand, the line sensor of the Si photodiode is very low (substantially zero) in quantum efficiency (sensitivity) to light in an ultraviolet to blue region and is high in quantum efficiency (sensitivity) to light in a red region, which results in a poor blue/red sensitivity ratio. Further since being large in dark current, the line sensor of the Si photodiode is not sufficient to detect weak stimulated emission in a blue region, and accordingly, an obtained image is very low in S/N ratio and in quality. Further, when a long line sensor such as of 17 inches is made of Si photodiode, the line sensor becomes very expensive. Further since the stimulated emission is very weak, it is necessary for the photoconductive material layer to be very high in dark resistance. However, the photoconductive material described above are all low in dark resistance and accordingly read-out must be effected with a relatively high electric field applied to the photoconductive material layer, which increases the dark current and makes it difficult to obtain a high S/N ratio.
Further there has been proposed, for instance, in Japanese Unexamined Patent Publication No. 60(1985)-236354, an image read-out method and system in which a stimulating light source which emits a spot light such as a laser beam, and a scanning optical system which moves the spot light and a line sensor relatively to the stimulable phosphor sheet in a sub-scanning direction are employed, and stimulated emission emitted from parts of the stimulable phosphor sheet exposed to the light spot is read in sequence while moving the stimulating light source and the line sensor relatively to the stimulable phosphor sheet in the sub-scanning direction. However the solid photoelectric convertor element forming the line sensor in this method and system is the same as that used in Japanese Unexamined Patent Publication No. 60(1985)-111568 and accordingly gives rise to the same problems.
In “RADIOGRAPHIC PROCESS UTILIZING A PHOTOCONDUCTIVE SOLID-STATE IMAGE” (772/Research disclosure, October 1992/34264), Japanese Patent Publication No. 7(1995)-76800 and Japanese Unexamined Patent Publication No. 58(1983)-121874, there is disclosed an image read-out system in which a stimulable phosphor sheet and a radiation image conversion panel which is substantially the same in area as the stimulable phosphor sheet, comprises a photoconductor material layer having sensitivity to the stimulated emission and sandwiched between a pair of electrode layers, and functions as a zero-dimensional photoelectric convertor are used and an image is read out while scanning the radiation image conversion panel with a spot light.
It is said that the photoconductor material layer is preferably of a photoconductive material which is high in sensitivity to the stimulated emission in a wavelength range of about 300 to 500 nm and low in sensitivity to the stimulating light in a wavelength range of about 600 to 800 nm. It is said that a preferable photoconductive material includes selenium compounds and amorphous (a-Se) is especially preferred.
However, use of selenium compound as the photoconductive material gives rise to the following problem. That is, though it is advantageous that the solid photoelectric convertor element itself has electron multiplying function since the stimulated emission is very weak, the S/N ratio cannot be high so long as the selenium compound has not electron multiplying function (the above identified references make no mention of whether the selenium compound has electron multiplying function). The selenium compound such as a-Se is not generally used for electron multiplication unlike the photomultiplier.
In “RADIOGRAPHIC PROCESS UTILIZING A PHOTOCONDUCTIVE SOLID-STATE IMAGE” (772/Research disclosure, October 1992/34264) (will be referred to as “reference 1”, hereinbelow), there is disclosed an image read-out system in which a stimulable phosphor sheet and a radiation image conversion panel which is substantially the same in area as the stimulable phosphor sheet, comprises a photoconductor material layer having sensitivity to the stimulated emission and sandwiched between a pair of electrode layers, and functions as a zero-dimensional photoelectric convertor are used and an image is read out while scanning the radiation image conversion panel with a spot light.
It is said that the photoconductor material layer is preferably of a photoconductive material which is high in sensitivity to the stimulated emission at 500 nm and low in sensitivity to the stimulating light at 633 nm. It is said that amorphous (a-Se) is especially preferable as the photoconductive material layer.
Since a-Se is highly sensitive to light not longer than 500 nm (e.g., a blue region from 300 to 500 nm) and is higher than a photomultiplier (as a zero-dimensional photoelectric convertor) in quantum efficiency to stimulated emission near 400 nm, a-Se is suitable for detecting stimulated emission emitted from the stimulable phosphor layer. Further since a-Se is hardly sensitive to light not shorter than 600 nm (e.g., a red region from 600 to 800 nm), and is large in the sensitivity to stimulated emission/sensitivity to stimulating light ratio, the a-Se photoconductive material layer can detect the stimulated emission emitted from the stimulable phosphor layer without use of stimulating light cut filter. Further, an a-Se layer can be formed by low-temperature deposition process, and is suitable for forming a solid image sensor which is thin, large in area and strong to impact.
However, when a radiation image conversion panel which is substantially the same in area as the stimulable phosphor sheet is formed of a-Se, the area of the photoconductive material layer becomes very large, which results in generation of an excessive dark current and a large capacitance (output capacity of detector), and the S/N ratio deteriorates.
Further, since it takes a long time for stimulated emission to be emitted from the stimulable phosphor layer upon exposure to the stimulating light, when the stimulable phosphor sheet is two-dimensionally scanned with stimulating light in the form of a spot beam, it takes a long time to read out image from the stimulable phosphor sheet.
Further, also in Japanese Patent Publication No. 7(1995)-76800 (will be referred to as “reference 2”, hereinbelow), it is disclosed that stimulated emission emitted from a stimulable phosphor layer is detected by a photoconductive material layer which is substantially the same in area as the stimulable phosphor sheet. It is said that the photoconductor material layer is preferably of a photoconductive material which is high in sensitivity to the stimulated emission in a wavelength range of about 300 to 500 nm and low in sensitivity to the stimulating light in a wavelength range of about 600 to 800 nm. It is said that selenium compounds are especially preferable as the material of the photoconductive material layer. It is further said that influence of a dark current can be suppressed by dividing a part of the electrodes into a plurality of electrode elements and detecting electric currents separately from the electrode elements.
However even if the electrode is so divided, the area of each electrode element is still large, and accordingly generation of an excessive dark current cannot be avoided and the capacitance is still large, which results in deterioration of the S/N ratio. Further, even if an image signal is read out by scanning the electrode elements with a spot beam, the read-out speed cannot be substantially increased.
Further also in Japanese Unexamined Patent Publication No. 58(1983)-121874 (will be referred to as “reference 3”, hereinbelow), it is disclosed that stimulated emission emitted from a stimulable phosphor layer is detected by a photoconductive material layer which is substantially the same in area as the stimulable phosphor sheet and the photoconductive material layer is formed selenium compounds. It is further disclosed that influence of a dark current can be suppressed by dividing a part of the electrodes into a plurality of electrode elements and detecting electric currents separately from the electrode elements. Further, the reference 3 further says that when the capacitance of the photoconductive material layer is large and additional noise is generated, the additional noise can be suppressed by dividing the electrode into a plurality of parallel stripe electrode elements.
However, even if the electrode is so divided, the electrode elements are not in one-to-one correspondence with the picture elements and signal read-out is not effected line by line. Accordingly, the read-out speed cannot be substantially increased. Further even if the electrode is so divided, so long as the pre-amplifiers such as current detecting amplifiers as a means for reading out the charges generated in the photoconductive material layer and obtaining an image signal are in the form of charge amplifiers, the output capacity itself forms a noise source. Further when the pre-amplifiers are of a current-voltage conversion system (including a logarithmic amplifier), it is difficult to ensure stability. In other words, it is difficult to obtain a high speed circuit. Further, not only the dark current but also residual electric charges can produce false signals or flare.
Further, there has been known a system in which “preliminary read-out” is effected prior to “final read-out” in order to ascertain the characteristics of the radiation image stored on the stimulable phosphor sheet such as the dynamic range of the radiation image. The preliminary read-out is carried out by use of stimulating light having stimulation energy of a level lower than the stimulation level of stimulating light used in the final read-out. On the basis of the preliminary read-out image signal obtained by the preliminary read-out, read-out conditions and/or image processing conditions for final read-out are determined.
Further we have proposed a method of determining the read-out conditions and/or image processing conditions for final read-out without carrying out such preliminary read-out. See Japanese Unexamined Patent Publication Nos. 55(1980)-48672, 55(1980)-50180, 56(1981)-11348, and the like. In this method, momentary light emitted from the stimulable phosphor sheet upon exposure to the recording radiation is detected by an exclusive detector such as a photo-timer, and information on the characteristic of the radiation image stored on the stimulable phosphor sheet, the amount of radiation stored on the stimulable phosphor sheet and the like is obtained on the basis of the detected momentary light, and the read-out conditions and the like are determined on the basis of the information.
The level of the stimulation energy means the total amount of stimulation energy to which the stimulable phosphor sheet is exposed (the amount of stimulation energy per unit time×time). In order to lower the level of the stimulation energy, exposed dose of the stimulating light is reduced, or the scanning speed is increased so that the number of picture elements becomes smaller as compared with the final read-out.
The read-out conditions are various conditions which affect the relation between the amount of stimulated emission and the output of the read-out system. The read-out conditions include, for instance, the read-out gain which governs the relation between input and output, the scale factor and the power of the stimulating light for read-out.
The image processing conditions are various conditions for carrying out processing which affects gradation, sensitivity and the like of an image reproduced on the basis of the image signal. In systems where the aforesaid preliminary read-out is not carried out, the image processing conditions include also the aforesaid read-out gain and the scale factor.
The method of determining optimal image processing conditions is applied to not only the system using the stimulable phosphor sheet but also the system in which an image signal is obtained from a recording medium such as X-ray film. The system for determining the read-out/conditions and/or the image processing conditions sometimes called an EDR processing system.
However, when the preliminary read-out is carried out by use of a low level stimulating light, the amount of information to be obtained by the final read-out is reduced by the amount of stimulated emission emitted from the stimulable phosphor sheet upon exposure to the preliminary read-out stimulating light, and accordingly, the image signal obtained somewhat deteriorates in the S/N ratio as compared with that obtained without carrying out preliminary read-out. Further, scanning the stimulating light beam for the preliminary read-out adds to the image read-out time.
On the other hand, when the image read-out conditions and/or the image processing conditions are determined by detecting momentary light emitted from the stimulable phosphor sheet upon exposure to the recording radiation by an exclusive detector such as a photo-timer, use of the exclusive detector adds to the cost. Further there is a problem that since the detector such as the photo-timer is generally narrow in detecting area and limited in measuring range, the conditions cannot be set at a high accuracy.